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acceleration of channel incision due to an increase in the eleva-tion of the range that must have occurred to form the canyons cut into the relict landscape that we observe today. This land-scape form can be uniquely associated with elevation increase simply because the downcutting of extremely deep valleys The non-equilibrium landscape into a low-relief landscape formed near sea level requires an increase in elevation. Changes in base level by 100 m due to of the Sierra Nevada, California:glacial/interglacial fluctuations are small on the scale of the total elevation change considered by our analyses (2500 m).M.K. Clark, G. Maheo, J. Saleeby, and K. Farley, California Our model assumes that upstream drainage area is a proxy Institute of Technology, Mailstop 100-23, Pasadena, California for discharge, which, among other things, assumes that pre-91125, USA, mclark@gps.caltech.edu cipitation is constant in space and time. It is possible that the modern Sierra Nevada receives more precipitation today than in early or mid-Cenozoic time due to an increase in orographic We thank H.F. Garner for calling attention to an important precipitation as the mountain range grew, or changes in mois-factor governing fluvial erosion: the role of variable stream ture yield due to atmospheric circulation and temperature discharge caused by climatic fluctuations. We agree that cli- changes. The change from a drier to more humid climate that matic variations affect erosion ...

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e11
doi: 10.1130/GSATOFe11
The non-equilibrium landscape
of the Sierra Nevada, California:
M.K. Clark, G. Maheo, J. Saleeby,
and
K. Farley,
California
Institute of Technology, Mailstop 100-23, Pasadena, California
91125, USA, mclark@gps.caltech.edu
We thank H.F. Garner for calling attention to an important
factor governing fluvial erosion: the role of variable stream
discharge caused by climatic fluctuations. We agree that cli-
matic variations affect erosion rates and stream morphology
by altering stream discharge, altering bed state such as armor-
ing of channel bottoms and changing sedimentary flux, and
can vary local base levels during glacial/interglacial cycles.
These processes have most likely played a role in changing
river profile form and erosion rates to some degree at various
times throughout the Cenozoic in the Sierra Nevada. However,
Garner argues that climatically driven changes in erosion rate
led to elevation change through isostatic adjustment without
any
need to call on tectonic forces to explain the modern
elevation of the range. This is where we disagree.
An increase in mean elevation of the range due to isostatic
adjustment to erosion alone requires incision of narrow river
valleys into a relatively unincised, elevated surrounding area
(e.g., England and Molnar, 1990; Whipple et al., 1999). Our esti-
mate of incision of river canyons into the relict landscape relied
on interpretation of longitudinal river profiles. Dominant phys-
ical erosional processes in bedrock rivers, such as plucking,
abrasion, and cavitation, along with other factors that control
channel incision rate, such as channel width, channel sinuosity,
and sediment supply, combine in complex ways that affect the
longitudinal channel profile; however, it has long been recog-
nized that longitudinal channel profiles exhibit a power-law
scaling relationship between local channel slope and contrib-
uting drainage area (e.g., Hack, 1973; Flint, 1974; Howard and
Kerby, 1983; Wobus et al., 2006). River profile reconstruction
and identification of relict landscape surfaces allowed us to
determine that the total volume of incision into the relict land-
scape has been small. While the limited magnitude of this inci-
sion undoubtedly drove some minor increase in elevation, it is
unlikely to have driven kilometers of isostatically-driven eleva-
tion change, even with a thin elastic lithosphere (e.g., Clark et
al., in press; Whipple et al., 1999).
Erosion-rate data support the idea that river channels are
responsive to both changes in rock uplift rate and changes in
precipitation (e.g., Snyder et al., 2000; Lave and Avouac, 2000;
Reiners et al., 2003; Wobus et al., 2003; Thiede et al., 2005).
We related changes in erosion rate in the Sierra Nevada to an
acceleration of channel incision due to an increase in the eleva-
tion of the range that must have occurred to form the canyons
cut into the relict landscape that we observe today. This land-
scape form can be uniquely associated with elevation increase
simply because the downcutting of extremely deep valleys
into a low-relief landscape formed near sea level requires an
increase in elevation. Changes in base level by 100 m due to
glacial/interglacial fluctuations are small on the scale of the
total elevation change considered by our analyses (2500 m).
Our model assumes that upstream drainage area is a proxy
for discharge, which, among other things, assumes that pre-
cipitation is constant in space and time. It is possible that the
modern Sierra Nevada receives more precipitation today than
in early or mid-Cenozoic time due to an increase in orographic
precipitation as the mountain range grew, or changes in mois-
ture yield due to atmospheric circulation and temperature
changes. The change from a drier to more humid climate that
Garner suggests could potentially have decreased the channel
relief on the relict landscape from mid-Cenozoic time to the
present. Therefore, using estimates of channel parameters from
the present relict landscape may underestimate predictions of
trunk stream paleo-relief. However, the lack of change in long-
term erosion rates recorded by helium ages and the agreement
between long-term and short-term erosion rates for the relict
landscape do not support a large-magnitude erosional event
that would be required to significantly reduce the relief on the
relict landscape. Also, the excellent agreement between the
independently calculated Kern and Kings river paleo-crestal
elevations, despite different orientations to prevailing wind
direction and differences in modern precipitation patterns,
argues that changes in precipitation are unlikely to have influ-
enced paleo-channel parameter estimates.
The notion that the Sierra Nevada has been undergoing
isostatic uplift due to erosional unloading since Jurassic time
stems from obsolete models of a thick felsic root having
formed beneath the batholith (Bateman and Eaton, 1967;
Carder, 1973; Pakiser and Brune, 1980). Average crustal
thicknesses in the southern Sierra Nevada are inadequate to
explain the high elevation of the range by simple Airy isos-
tasy (Fliedner et al., 1996, 2000). Slow seismic wavespeeds
beneath the range, changes in volcanic chemistry, and petro-
logic changes in deeply sourced xenoliths all point to a major
change in deep lithospheric composition in late Miocene or
Pliocene time that would have led to a decrease in the average
density of the lithosphere and a resulting elevation increase
(Fliedner et al., 1996, 2000; Ducea and Saleeby 1996, 1998;
Manley et al., 2000; Farmer et al., 2002). The regionally consis-
tent pattern of low relief, except in glaciated areas, on the sub-
Eocene relict landscape and its slow denudation as recorded
in our helium data further refutes the notion of kilometers of
isostatic uplift due to erosional unloading since Jurassic time.
We suggest that all of the available evidence points to a pre-
dominately tectonic, rather than climatic-isostatic, source of
elevation change for the southern Sierra Nevada during late
Cenozoic time.
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Manuscript accepted 27 January 2006; published online April
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